1. Field of the Invention
This invention relates to fuel nozzles of the type that employ an entrainment fuel nozzle for initial, partial premixing of gaseous fuel and air. Such structures of this type, generally, use the gas jet to entrain surrounding air so that the fuel is somewhat diluted prior to injection into the combustor.
2. Description of the Related Art
The foremost combustor concept for low NOx emissions, lean premixed combustion, requires that fuel and air be well premixed prior to ignition. In practice, and particularly when retrofitting existing machines, this high degree of premixing is difficult to achieve, resulting in localized fuel-rich regions and the associated NOx generation. For example, FIG. 1 illustrates a conventional combustor 2. Combustor 2 includes, in part, casing 4, air inlet 6, liner 8, dilution holes 9, combustion chamber 10, fuel spoke 14, flow sleeve 16, swirler 18, and exhaust 20.
During the operation of a conventional premixed combustor 2, air enters in through inlet 6 along the direction of arrow A and enters into casing 4. Combustion air then enters combustion chamber 10 through swifter 18 along the direction of arrows B'. While air enters combustion chamber 10, gaseous fuel is injected through fuel spokes 14 along the direction of arrows D. This fuel is then swirled by swifter 18 and enters combustion chamber 10. As the air and fuel enter into combustion chamber 10, they are combusted. After they are combusted, dilution air mixes in through dilution holes 9 along the direction of arrows B. The additional air which enters from B serves primarily to drop temperature after combustion is complete. After the fuel and air are combusted and mix with the dilution air they are exhausted through exhaust 20 along the direction of arrow E.
The manner in which fuel and air are introduced into combustion chamber 10 generally results in poor mixing prior to combustion. As discussed earlier, these fuel-rich regions result in increased NOx production. Although efforts have been made to effect better premixer and swifter designs in order to achieve increased mixing and reduced NOx, it would also be desirable to dilute the fuel concentration prior to injection into the combustion chamber to achieve partial premixing of the fuel within the fuel nozzle. This partial premixing would further reduce the fuel concentration within the fuel rich regions in the combustion chamber. Such premixing and the associated NOx reduction would be particularly advantageous in situations where major modifications of the combustion system to a fully premixed design are not possible or practical.
As can be seen in FIG. 2, premixing air into the fuel line of a conventional diffusion combustor of the type shown generally in FIG. 1 results in a drop in NOx emissions. NOx emissions in the combustor exhaust are shown to decrease as air is mixed into the gaseous fuel prior to the nozzle and introduction into the combustor. This drop is larger than can be simply explained by a drop in the overall air/fuel ratio, as the combustor exit temperature is held constant. Diluting the fuel with air prior to combustion increases mixing and reduces NOx production. However, presently there are no combustors which exhibit this premixing and dilution of the fuel within the fuel nozzle.
Obstacles to this technology have, up to this point, included the need for an additional air compressor (particularly difficult for a retrofit), added control complexity, and the danger of creating a combustible or explosive mixture outside the combustion chamber 10. Therefore, a more advantageous combustor, then, would be presented if there would be an initial premixing of the fuel in air in a more simplified manner.
It is apparent from the above that there exists a need in the art for a combustor which is able to premix the fuel and air, and which at least equals the combustion characteristics of the known combustors, but which at the same time is capable of premixing the fuel and air prior to the fuel and air being injected into the premixing chamber. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.